The prior art as concerns peat treatment is described extensively in the report from the Swedish Board for Research in Energy Production No. NE 1981:5 and in the Peat Report of The National Swedish Power Administration 1982. The latter report illustrates different peat production methods in a figure on page 4:2. Conventionally, peat is mined either by milling or by cutting. Both these mining methods are dependent on weather, which constitutes a very substantial drawback. The mining season can be extended rather considerably by digging up the peat or by applying the so called hydro-peat method. Neither of these methods makes possible year-round mining, since they do not provide an economically feasible technique to melt frozen peat in the wintertime.
Fractioning is one of the pre-treatment methods. Thus, it is known that dewatering peat by pressing may be considerably simplified if the fine parts fraction, which is difficult to dewater, is separated from the more easily dewatered fibrous fraction. Pre-treatment of peat by such a method, fractioning, may be carried out mechanically in a number of different ways according to known art, e.g. by using sieves, or centrifuges, or by applying the so called Float-wash technique, etc. These known methods make it possible to raise the concentration of dry matter in a pumpable peat suspension to the level of about 10%. These techniques, however, presuppose the absence of ice in incoming peat, which limits the time of use for this equipment. It is not economically feasible to supplement this equipment with additional devices for heating the raw peat in order to melt the ice. Continuous production independent of time of year is therefore not possible, which constitutes a considerable limitation for these systems.
It is also known since a long time that peat constitutes a colloidal system with a strong affinity for water, and that the water which is colloidally bound to the peat will most easily be removed by pretreatment with additions of different electrolytes. Especially that water which is contained in the colloidal fine parts fraction of the peat is bound in this manner, and it has therefore been considered of interest to find simpler methods or additions, so that this part of the fraction may also be dewatered mechanically. Addition of polyelectrolyte is rather costly. This is especially true when the peat contains a large fraction of fine parts, which is the case when the peat is of the highly humified type and/or if some of the fine parts fraction is recirculated in the system, so that the concentration of fine parts in incoming raw peat is increased.
Methods of pressing are known which will raise the dry matter content to about 30%. Several types of presses have been tested according to the report NE 1981:5 previously mentioned, among them a so called multi belt press in combination with a roller press. A dry substance content of about 30% may be achieved by this known technique. Economic considerations based on press experiments indicate that pressing should not be required to give higher dry substance content than this. To reach higher levels of dry substance content long pressing times are required, which reduces the output of the process considerably. This in turn makes the cost exceedingly high. Therefore still another dewatering step must be applied, namely drying. It is, however, expensive to dry peat which contains as much water as 70%. It is therefore desirable to increase the dry substance content prior to drying, which is not economically possible by applying known art. Therefore, it cannot be stated that known methods of pressing cooperate with the other parts of the equipment in a way which is synergistic to the whole system.
Several methods and devices have been developed for drying. One method is described in SE No. 78 10558-2. According to this method the material is dried in a steam drier, where the material is surrounded by high pressure steam. The steam is genererated in a separate boiler, which makes the system rather costly and makes it unrealistic at least for small or medium plants. From this point of view SE No. 78 06720-4 describes a more advantageous method, which however requires an apparatus which only works with direct drying at atmospheric pressure, precluding a high efficiency.